test(fadd): avoid potential in-consistency

tests/fadd.bats

@@ -54,11 +54,11 @@ stage_modified_file() {
 @test "fadd stage selected dir" {
   run stage_selected_dir
   [ "${status}" -eq 128 ]
-  [ "${lines[0]}" = "fatal: pathspec 'searchdir' did not match any files" ]
+  [[ "${output}" =~ "fatal: pathspec 'searchdir' did not match any files" ]]
 }
 
 @test "fadd stage modified file" {
   run stage_modified_file
   [ "${status}" -eq 128 ]
-  [ "${lines[0]}" = "fatal: pathspec '$HOME/modifiedfile' did not match any files" ]
+  [[ "${output}" =~ "fatal: pathspec '$HOME/modifiedfile' did not match any files" ]]
 }